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London was hotter than the Caribbean, the Western Sahara and popular holiday destinations in Europe on Monday - with even hotter weather on the way on Tuesday.
The Met Office forecast the capital could see highs around 40C this week as the heatwave sweeps across England.
On Monday afternoon the highest recorded temperature was 37.5C at Kew Gardens in west London, according to the Met Office.
The rise in temperatures has forced the UK Health Security Agency to issue a level 4 heat-health alert – described as an “emergency” – while the Met Office has issued the UK’s first red extreme heat warning, with both running from Monday to Wednesday.
The capital’s scorching temperatures mean it was warmer than Nassau in the Bahamas (32C), Kingston in Jamaica (33C), Malaga in Spain (28C), Athens in Greece (35C), Albufeira in Portugal (28C) and Dakhla, in the Western Sahara (24C).
Network Rail is urging people in England to only travel if absolutely necessary on Monday and Tuesday - and to expect delays and cancellations due to the heat affecting services.
Britain’s railways are “simply not engineered” to cope with the temperatures expected during this week’s heatwave, experts added.
Network Rail route director Gary Walsh said: “The railway in this country is simply not engineered to run normally in such extreme temperatures.
“We’ve got extra teams out around the route, ready to respond to any incidents, but with temperatures set to break all records, we need to also reduce the speed at which trains can run.
“This reduces the risk of tracks buckling in the heat and also means that when the overhead line expands it won’t get tangled on passing trains.”
Transport for London said around 1.06 million entries and exits were made by London Underground passenger on Monday up to 10am.
This is down 18% compared with the same period last Monday.
Some 1.07 million bus journeys were made up to 10am, a 10% decrease week on week.
TfL said: "Ridership on Monday is typically lower than other days of the week on public transport and is therefore likely to be a good indication of where people are working from home.
"Typically, TfL also sees a small reduction in ridership at this time of year as schools enter their last week of term and people begin to go on holiday.
"However, the recent high temperatures have led to more of a reduction than would have been expected before our travel advice was issued to only make essential journeys during this extreme hot weather."
Above: No air conditioning means a hot journey for commuters on the Central Line
The temperatures are forecast to hit a high in the capital on Tuesday, 19 July, during a week expected to bring exceptionally high temperatures across the UK.
There is around a 30% chance that the current national heat record – of 38.7C set in Cambridge in 2019 – could be broken.
The Met Office added on Friday that there is a 80% chance of the heat record being broken, and a 50% chance of the highest temperatures exceeding 40C, according to guidance.
The forecaster issued the rare red warning due to concerns that the 'extreme heat' conditions could be a danger to health, particularly for vulnerable and elderly people.
How is London responding to the red heat warning?
Authorities in the capital are already preparing for heatwave disruption to transport and key infrastructure.
London Mayor Sadiq Khan is urging Londoners to be "very careful" in the "potentially dangerous" conditions.
A meeting of London’s Strategic Communication Group has been called today as the capital prepares for the extreme heat.
On Thursday, the Mayor triggered a severe weather emergency response to support rough sleepers struggling in soaring temperatures, to provide them with safe accommodation.
This response will continue throughout the hot weather period, City Hall says.
London Ambulance Service has urged the public to support it as the heat continues by only calling 999 in the event of a life-threatening emergency, keeping hydrated, and staying out of the sun during the hottest periods of the day.
Will London transport be disrupted?
Commuters are being warned to brace for heat impact on trains and Tube networks as the mercury soars this week.
Downing Street has said speed restrictions on rail lines may be required under contingency plans nationally, as temperatures rise.
A No 10 spokesman said discussions with sectors including the NHS will “continue to work closely with all of those sectors.”
“Specialist teams from Network Rail and TfL (Transport for London) will continue to monitor the impact of high temperatures to try and make sure they can keep services running safely and reliably for customers,” he added.
“It may be the case that speed restrictions are likely to be put in place on some parts of the network to manage the hot weather and to avoid any potential damage.
“DfT (Department for Transport) are engaging with port operators and highways agencies, as well as the police, to make sure plans are in place particularly in places where you may see queuing.”
This week, Hammersmith Bridge in west London was wrapped in foil to protect it from the heatwave whilst the Grade II listed Victorian span undergoes strengthening works.
Engineers installed chains with silver insulation to protect the bridge, so it can remain open during the hot weather period.
Mr Khan said all Londoners should expect disruption during the red warning period. “This warning is not limited to the most vulnerable and means that substantial changes in all our daily routines will be required, and there is likely to be an impact on travel.
He continued: “All of the capital’s key agencies and emergency services are working closely together to ensure London is prepared for the extreme heat that is forecast and will continue to provide advice for Londoners ahead of Monday.
“I urge Londoners to be very careful and to look after each other in the comings days.”
Why is it getting so hot in London?
Heatwaves have been made hotter, longer and more frequent by climate change, and experts have warned of the need to adapt homes and cities in the UK for a future of more intense summer heat.
The UK’s record high for 2022 currently stands at 32.7C, recorded at Heathrow, west London, on 17 June.
Met Office chief meteorologist Paul Gundersen said: “Exceptional, perhaps record-breaking temperatures are likely, quite widely across the red warning area on Monday, and focused a little more east and north on Tuesday.
"Currently there is a 50% chance we could see temperatures top 40C and 80% we will see a new maximum temperature reached.
“Nights are also likely to be exceptionally warm, especially in urban areas. This is likely to lead to widespread impacts on people and infrastructure. Therefore, it is important people plan for the heat and consider changing their routines. This level of heat can have adverse health effects.”
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